1. Field of the Invention
The present invention generally relates to a packet switched network. More particularly, the present invention relates to a method of dynamically metering packet flow in a packet switched network to reduce or avoid packet loss.
2. Discussion of the Related Art
Ethernet local area networks (LANs) offer a hardware driven method of flow control based on a receive first-in-first-out (FIFO) “fullness” threshold. This Ethernet flow control standard is covered in the Institute of Electrical and Electronics Engineers (IEEE) 802.3x Standard specification. The Ethernet flow control standard is intended to reduce FIFO overruns, which result in packet loss. Packet loss reduces the network reliability and reduces available bandwidth.
In Ethernet flow, a media access control (MAC) device/receiver (e.g., any network device, endpoint, or infrastructure, such as a server, client, switch, router, bridge, etc.) receives packets, the packets are placed into a FIFO buffer. The packets are processed, and the FIFO space that they once occupied is then made available for subsequent packets. Processing a packet generally includes direct memory access (DMA) transfer of the packet to system memory, or forwarding it onto another MAC device to be transmitted, such as another port or switch. Packet processing may additionally include tasks such as checksum verification or decryption operations. If the rate of incoming packets exceeds the rate at which the MAC device can process them, the MAC device's receiver FIFO begins to fill. When a fullness threshold is exceeded, the MAC device sends a pause packet to its link-partner. A pause packet or frame causes the link-partner to momentarily stop transmitting to the receiver. This pause allows the receiver time to process the already received packets in the FIFO and prevents the receiver's FIFO from overflowing with subsequent packets.
The IEEE 802.3x Standard flow control does not completely prevent packet loss. Even in a back-to-back two system network, packet loss is not completely prevented. First, the controllers only transmit a pause frame after a fullness threshold in the receive FIFO has been reached. However, the pause frame cannot be always sent immediately after the threshold has been reached. To avoid other protocol violations, the pause frame can be only sent after the controller completes transmitting the current frame, if any, already being sent. The pause frame then propagates across the media. Once the sending station receives the pause frame, it processes the frame before it can respond. If the sending station is in the middle of sending a packet, this packet transmission must be completed before the pause request is honored. All of these delays may result in several kilobytes of data arriving after the threshold value is crossed. This occurrence can result in overrun and packet loss.
Additionally, even if the sending station heeds the pause request promptly and ceases sending packets before an overrun occurs, the pause is only momentary. There is no guarantee that the receiver will be able to free up FIFO space and make room for subsequent packets during this momentary pause. For example, the receiver may be waiting for access to a shared bus, such as a peripheral component interconnect (PCI) bus, to transfer packets to the system memory. If the bus is heavily loaded, the controller may not be granted access before the pause time expires. In this case, the receiver's FIFO would not drain to host memory to make room for subsequent packets while the sender was pausing. After the pause, the sender may continue sending packets that overrun the receiver's buffering capabilities.
The above-mentioned conditions may occur even in a simple back-to-back connected network, or with two personal computers (PCs). If one or more switches and several clients are in the network, then the intrastate devices may drop packets as well. Pause frames do not propagate, and so a switch may be forced into a situation where it is receiving packets destined for an end station to which it is not allowed to forward packets, due to receiving a pause frame from that station. This error will quickly consume the switch's buffering capabilities.
Accordingly, there is a need for a flexible method of monitoring packet flow to prevent and reduce packet loss while utilizing existing protocols.